Method of treating perbles



Aug. 8, 1944.

F. C. MCGAUGHY METHOD OF TREATING PEBBLES Filed July l5, 19.42

. tirely satisfactory for grinding purposes.

Patented Aug. 8, 1944 Umrao STATES PATENT ori-ica Franklin C. McGaughyg, San Antonio, Te". Application July 13, 41942, serial No. 450,72

\ 9 Claims.

Generically, the present invention relates to grinding media, and specifically it hasv reference to methods for treating natural flint pebbles, boulders or the like of cretaceous origin so that the resultant products may be used as grinding media in pebble mills, etc.

There are no natural unprocessed iiint grinding pebbles since such pebbles must possess the qualities of abrasive resistance, fracture and breakage resistance, non-contaminating properties and a certain minimum roundness. Heretofore, French and Danish pebbles have been uniformiy used as they possess the above-mentioned properties. However, the French and Danish pebbles are not unprocessed ones as they have been treated by tidal action, attrition and abrasion. The only vdeleterious substances present in these pebbles is a substantially small amount of ocean water that can be readilyremoved by superiicial cleaning and washing. By reason of their 'great hardness, the French and Danish pebbles are recognized inthe arts as the standard non-ferrous pebble.

In view of the iact that these pebbles have-a1- ways been imported from Europe, it will be appreciated that the current world conditions make it extremely diiiicult to obtain these pebbles-in this vide grinding pebbles that are comparable with either the French or Danish pebbles from both the economic and utilitarian standpoints.

' These deposits have a deleterious outer coating of either lime or weathered silica that must be removed completely if the pebbles are to be en- The pebbles have an increasing abrasive resistance under the outer coating toward the inner core which-I have determined possesses great hardness. As a consequence, by being able to remove expeditiously the outer coating, it is possible to provide a non-ferrous pebble having excellent abrasive qualities.

The present invention is, of course, primarily concerned-with treating ilint vboulders or gravels in such a way that the previously undesirable pebbles may be made available as grinding media at a reasonable cost. While I prefer to process the. raw products by a grinding action in a pebble mill or other type of cylinder mill to remove the outer coating, this can be accomplished in other ways, such as, by chemical'treatment with acids or strong alkalis, by sand or shot blasting, by a scouring action with an abrasive grain, or by vibrations of an hydraulic jack hammer. By the term cylinder mill I mean to include those types of machines that consist essentially of hollowl containers of circular cross section mounted with the axis substantially horizontal, and partially filled with particles of material that are caused to tumble under the influence of gravity by revolution of the container.

Accordingly, one important object of the present invention is to provide a method wherein flint -deposits of cretaceousl origin are so treated that the resultant products find particular application for u se as grinding pebbles in various types of mills using both ferrous and non-ferrous grinding media. y

Another object of this invention is to treat4 iiint deposits of cretaceous origin so that the p ebbles 'produced thereby are comparable with French and Danish pebbles which are recognized as the standard non-ferrous grinding pebble.

Yet a further object is to provide a vnon-ferrous grinding pebble from continental flint deposits which possesses excellent abrasive resist-` ance, fracture and breakage resistance, non-contaminating qualities and a certain -minimum roundness. l

And yet a further object of my present invention is to provide a treatedcontinental flint pcbble of cretaceous'origin whose minimum roundness and abrasive lresistance is at least equal to or more than that of the French and ,Danish pebbles.

To accomplish the above and other advantageous objects, the present invention in its broadest aspect comprises subjecting raw, unprocessed continental flint gravels or pebbles to 'a step whereby the outer deleterious coating is removed and a pebble having an inner -core of high abrasive resistance is provided. More specifically, in the preferred embodiment ofmy inventiomthe raw, untreated pebbles are admitted into a rotating mill through which a stream of water is continuously. flowing and in whichlthe pebbles are subjected to a severe grinding operation thus rapidly removing the detritus which is-washed away by the water flowing through the mill. After a. predetermined period, the speed of the mill is reduced and by running the mill at the reduced speed, the pebbles will be subjected to an attrition action that closely simulates the tidal action thereby giving a polish to the outer surfaces of the pebbles.

In the drawing, Figure 1 is a longitudinal sectional View of a conical pebble mill showing the grinding action to which the gravel is sub-v jected.

Figure 2 is a transverse sectional view of the mill shown in Figure l illustrating the severe grinding operation..

Figure 3 is a transverse sectional view of the mill illustrating the less severe grinding to which the pebbles are subjected for polishing off the.

fractured outer surface. y

Figure 4 is a transverse sectional view of a raw, untreated flint pebble or boulder showing the deleterious substance that'must be removed.

Figure 5 is a view'sirnilar tov Figure 2 showing the deep fractures in the outer surface of the pebble or boulder after the severe grinding step.

Figure 6 isa view similar to Figures 2 and 3 illustrating the pebble after it has been fully treated and is available for use as a grinding pebble. v

As previouslyl indicated, I prefer to treat the raw gravels or flint pebbles or boulders (Figure 4) in a mill suchas an 8' by 30" Hardinge cone mill which I have illustrated generally by the numeral l. The mill has an inlet. 2, outlet 3, side door 4 and is driven through gear'S. The raw, untreated pebbles are introduced into the mill and a stream of water is admitted through a pipe 6. After the pebbles have been admitted, the mill is rotated at 24 or 25 revolutions per .nomic standpoints.

Or, again. either sand or Carborundum can be injected into the Hardinge mill to accomplish this same result.

By virtue of the present invention, I have made available for use as grinding media, a. raw, untreated product which in the past has been considered unsuitable forV use. After the removal of the deleterious outer coating, the pebble has an increasing abrasive resistance, which'means that the inner core has great hardness. My process can be very easily and inexpensively car- -ried out and the ultimate product is one which will compare favorably with the recognized standard of non-ferrous grinding pebbles,'both from abrasive and wear resistance and from the eco- It is definitely superior to 1 any non-ferrous pebble with the exception of the minute, the side door 4 being partially opened in order that the water and detritus may flow freely therefrom when the mill is in the position with the door on the underside. The mill is rotated at this speed for approximately 3 hours (see Fig. 2), at the termination of which, all of the outside coating is either ground completely off or loosened by Virtue of the rather severe impact grinding to which the gravel is subjectedv (see Fig. 5). A particularly important feature is that the flow of water through the mill. will remove immediately the detritus eliminating the cushion effect of the detritus and causing a more severe.

and rapid grinding action.

After the mill has been run in the neighborhood of 3 hours at 24 or 25.revolutions per minute, the speed is then reduced to approximately 20 revolutions per minuteJ At this speed, and attention is Icalled to Figure 3, the pebbles settle down against the inner walls of the mill and in lieu of the severe impacting, the pebbles will ride down the cascade of the mass whereby they are subjected to a less severe impacting and to more attrition or grinding within the mass. The mill Jis run at this speed to remove the fractured sur-l face caused by the severe grinding thereby giving a polish to the exterior surface of the pebbles, and Figure 6 illustrates this nal product.

'I'he length of time that the mill is operated at 20 revolutions per` minute is' largely dependent upon the depth of the fracture, but I can say with certainty, that if a finished pebble is desired for ner grinding, the mill should be run for approximately 2 hours at the lower speed.

French and Danish pebbles as illustrated by the following table:

In the specification, the'v notation flintpebbles refers to crypto-crystalline forms of silica; pebbles of continental cretaceous origin refers to flint pebbles or gravels of cretaceous origin, but whose deposition in a later, period was continental and are now still in continental deposits.

I desire that my invention be limited only by the appended claims and the showing of the prior art.

I claim:

1. A method of treating cretaceous flint pebbles taken from a continental deposit to make them available for use as grinding media for pebble mills comprising the steps of introducing the pebbles into a mill, rotating the mill rapidlyv prising subjecting the raw pebbles to an impact force thereby to remove the deleterious outer layer from the pebbles, and then subjecting the pebbles to a rubbing action to remove surface fractures from the pebbles byv attrition. f

3. A method of treating flint pebbles take from a continental deposit to render them suitable' for grinding media for pebble mills comprising subjecting the raw pebbles to an impact Instead of employing the same mill for the.

polishing step, it is sometimes worth while to puse a smallervmill which has a Carborundum lining.

force thereby to remove the deleterious outer layer from the pebbles, and then subjecting the 'having substantially rounded contours and substantially equal resistance to abrasion and fracture.

6. A method of treating cretaceous flint pebbles taken from a continental deposit to render them, suitable for use as grinding media for pebble mills comprising the steps of introducing a batch of said pebbles into a cylinder mill, rotati'ng the mill to thereby remove the deleterious outer layer by interimpaci"l of the pebbles, and continuously flushing the mill with water to continuously remove detritus therefrom.

7. A method of treating cretaceous ilint pebbles taken from a continental deposit to render them suitable for use as grinding media for pellible mills comprising the steps of introducing "a batch of said Pebbles into a cylinder mill, rotating the mill to thereby remove the deleterious outer layer by interimpact of the pebbles, continuousl'y introducing a ilushing liquid to the milLthrougli a trunnion thereof, and'continuously discharging said liquid peripherally of the mill to flush the mill of detritus.l

8. A pebble for use in pebble mills consisting of a cretaceous flint pebble from a continental deposit which has been articially reduced to a. core portion of uniform hardness throughout.

9. A grinding material for use in a pebble mill consisting of a batch of cretaceous flint pebbles taken from a continental deposit, the individual pebbles having been artificially reduced to substantially rounded core portions thereof having substantially equal resistance to abrasion and go' fracture.

FRANKLIN c. M'CGAUGHY. 

